Field of the Invention
The present invention relates to a catalyst containing catalytic metal supported on titanium oxide, and a method of producing the catalyst.
Background Art
In various fields ranging from catalytic chemistry to semiconductors, research has been conducted to use metals or metal oxides prepared in the form of nanoparticles. Nanoparticles refer to particles on the scale of nanometers. With their size characteristics, nanoparticles have functions and effects greatly differing from the same material in bulk form. For example, nanoparticles have higher activities due to their larger specific surface areas, and have quantum size effects.
To show such unique functions, nanoparticles need to have their size and shape under control. However, nanoparticles are physically and chemically unstable. For example, their size and shape can change when, for example, neighboring nanoparticles aggregate. Controlling such aggregation and other behaviors of nanoparticles has been difficult.
In catalytic chemistry, for example, catalysts with high activities are produced by preparing nanoparticles of metal as a catalyst, which are then dispersed and supported onto the surface of a catalyst carrier. When this catalyst is used under high temperature conditions intended for a catalytic reaction or under high temperature conditions caused by heat generated from a catalytic reaction, the heat from such intentional heating or the reaction heat from the catalytic reaction can move the catalytic metal dispersed and supported on the catalyst carrier. The catalytic metal may then undergo aggregation and sintering. As a result, the nanoparticles may change their size and shape, and may no longer have their properties unique to nanoparticles, or may have smaller specific surface areas. This will greatly lower the catalytic activity of the catalyst.
In response to this issue, Patent Literature 1 describes a catalyst for hydrogen reduction, which contains metal catalyst nanoparticles with a particle diameter smaller than 10 nm supported on catalyst carrier powder.
A catalyst for hydrogen reduction produced with the technique described in Patent Literature 1 can accelerate a hydrogenation reaction of carbon dioxide with high conversion under atmospheric pressure at a reaction temperature not exceeding 200° C.
However, reactions using catalysts for hydrogenation, such as catalytic reactions for converting carbon dioxide into methane or methanol and reductive reactions of exhaust gas, commonly involve high temperature conditions ranging from 300 to 600° C. If the catalyst for hydrogen reduction described in Patent Literature 1 is used under such high temperature conditions, catalytic metal supported on the catalyst for hydrogen reduction may be sintered to increase their particle size and may no longer be nanoparticles, or may have smaller surface areas. This will greatly lower the catalytic activity of the catalyst. The catalyst for hydrogen reduction described in Patent Literature 1 thus cannot be used under high temperature conditions.